This invention relates to the casting of steel strip and particularly to casting of steel strip using roll casters.
In a roll caster, molten metal is cooled on casting surfaces of at least one casting roll and formed into thin cast strip. In roll casting with a twin roll caster, molten metal is introduced between a pair of counter rotated casting rolls that are cooled. Steel shells solidify on the moving casting surfaces and are brought together at a nip between the casting rolls to produce a solidified sheet product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region in which the casting rolls are closest together. In any case, the molten metal is usually poured from a ladle into a smaller vessel, from where it flow through a metal delivery system to distributive nozzles located generally above the casting surfaces of the casting rolls. In twin roll casting, the molten metal is delivered between the casting rolls to form a casting pool of molten metal supported on the casting surfaces of the rolls adjacent to the nip and extending along the length of the nip. Such casting pool is usually confined between side plates or dams held in sliding engagement adjacent to ends of the casting rolls, so as to dam the two ends of the casting pool.
When casting thin steel strip with a twin roll caster, the molten metal in the casting pool will generally be at a temperature of the order of 1500° C. and above. It is therefore necessary to achieve high cooling rates over the casting surfaces of the casting rolls. High heat flux and extensive nucleation on initial solidification of the metal shells on the casting surfaces is needed to form the steel strip. U.S. Pat. No. 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial portion of the metal oxides formed are liquids at the initial solidification temperature, and provide high heat flux during the casting campaign. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, formation of the steel shells and strip can be influenced by the texture of the casting surface.
When casting steels in a thin strip casting process, manganese, silicon, chromium and aluminum are typically present at elevated oxygen levels. There is a tendency for the steel composition and slag composition to react with the refractoried used for the molten metal delivery system to distribute the liquid steel along the casting rolls. Specifically, the core nozzles and other refractory components are usually produced from a refractory material, such as alumina or zirconia combined in some proportion with a carbon source. The reaction of steel/slag compositions with the refractories produces carbon monoxide (CO) as a reaction product. The carbon monoxide gas formed as a result of the reaction disturbs the liquid steel pool just prior to solidification and forms waves on the surface of the molten metal in the casting pool. This disturbance can then be solidified in the strip and produces a defect referred to as a meniscus mark. Meniscus marks are defects that manifest as cracks on the steel strip surface. Meniscus marks are illustrated in FIG. 1.